In the anti-spam industry, spammers use various creative means for evading detection by spam filters. Accordingly, spam filter designers adopt a strategy of combining various detection techniques in their filters.
Current tools for message sender analysis include IP blacklists (e.g., real-time blacklists (RBLs)) and IP whitelists (e.g., real-time whitelists (RWLs)). Whitelists and blacklists add value to the spam classification process; however, whitelists and blacklists are inherently limited to providing a binary-type (YES/NO) response to each query. In contrast, a reputation system has the ability to express an opinion of a sender in terms of a scalar number in some defined range. Thus, where blacklists and whitelists are limited to “binary” responses, a reputation system can express “shades of gray” in its response.